An automatic injector is a device that performs intramuscular or subcutaneous administration of a therapeutic agent. An advantage of automatic injectors is that they contain a measured dose of a therapeutic agent in a sealed sterile cartridge. Automatic injectors can therefore be used in emergency situations to quickly and simply inject the therapeutic agent without having to measure dosages. Another advantage of automatic injectors is that the administration of the therapeutic agent is accomplished without the user initially seeing the hypodermic needle through which the therapeutic agent is delivered. Still another advantage is that the user does not have to manually force the needle into the patient. This is particularly advantageous when the therapeutic agent is being self-administered.
In some automatic injectors, the therapeutic agent is stored as a liquid solution. However, the long-term storage of a therapeutic agent in liquid term is problematic. For instance, some therapeutic agents in liquid form are not stable and thus have a shorter shelf-life than their solid counterparts. To address this concern, automatic injectors have been developed that store the therapeutic agent in solid form and, immediately prior to injection, mix the solid therapeutic agent with a liquid injection solution also stored in the injector. Such devices are generally referred to as wet/dry injectors. An example of such an injector is found in U.S. Reissue Pat. No. RE 35,986, entitled “Multiple Chamber Automatic Injector,” the disclosure of which is incorporated herein by reference. These injectors require the user to manually rupture a sealing member between the solid and liquid components and then manually shake the injector body to expedite dissolution of the solid component prior to injection. Unfortunately, steps such as manually shaking the injector increase the time needed to administer a dose of the therapeutic agent, which is undesirable in many emergency medical situations where rapid delivery of the therapeutic agent is needed (e.g., in nerve gas and chemical agent poisoning).
Additionally, many of the wet/dry devices available are not capable of storing or delivering a small, precise amount of a therapeutic agent even though many therapeutic agents are effective at very small doses. For example, a therapeutically effective dosage of epinephrine may be about 0.1 mg to about 0.5 mg. In known wet/dry devices, however, the dry chamber that accommodates the therapeutic agent is relatively large. In order to effectively fill the dry chamber to ensure effective mixing, a greater amount of the therapeutic agent should be loaded into the chamber, which is costly and potentially dangerous to the person receiving the injection. Alternatively, various excipients may be added to the therapeutic agent to effectively fill the dry chamber. However, adding excipients to a therapeutic agent requires additional expense and manufacturing in order to formulate and produce the therapeutic agent with the excipients prior to the agent being loaded into an injector.
Therefore, a need exists for a cost-effective automatic injector that can store a small, precise amount of a therapeutic agent and that does not require manual premixing by the user.